6TiSCH Operation Sublayer Protocol (6P)
Univ. of Sci. and Tech. Beijing 30 Xueyuan RoadBeijingHebei100083Chinawangqin@ies.ustb.edu.cnUniversitat Oberta de Catalunya156 Rambla PoblenouBarcelonaCatalonia08018Spainxvilajosana@uoc.eduAnalog Devices32990 Alvarado-Niles Road, Suite 910Union CityCA94587USAthomas.watteyne@analog.comInternet Area
6TiSCHDraft
This document defines the IPv6 over the TSCH mode of IEEE 802.15.4e (6TiSCH) Operation Sublayer (6top) Protocol (6P), which enables distributed scheduling in 6TiSCH networks.
6P allows neighbor nodes to add/delete TSCH cells to one another.
6P is part of the 6TiSCH Operation Sublayer (6top), the next higher layer to the IEEE Std 802.15.4 TSCH medium access control layer.
The 6top layer terminates the 6top Protocol defined in this document, and runs one or more 6top Scheduling Function(s).
A 6top Scheduling Function (SF) decides when to add/delete cells, and triggers 6P Transactions.
This document lists the requirements for an SF, but leaves the definition of SFs out of scope.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
All communication in a IPv6 over the TSCH mode of IEEE 802.15.4e (6TiSCH) network is orchestrated by a schedule .
The schedule is composed of cells, each identified by a [slotOffset,channelOffset].
This specification defines the 6TiSCH Operation Sublayer (6top) Protocol (6P), terminated by the 6TiSCH Operation sublayer (6top).
6P allows a node to communicate with a neighbor node to add/delete TSCH cells to one another.
This results in distributed schedule management in a 6TiSCH network.
The 6top layer terminates the 6top Protocol, and runs one or more 6top Scheduling Functions (SFs) that decide when to add/delete cells and trigger 6P Transactions.
The SF is out of scope of this document but this document defines the requirements for an SF.
The example network depicted in is used to describe the interaction between nodes.
We consider the canonical case where node "A" issues 6P requests to node "B".
We keep this example throughout this document.
Throughout the document, node A always represents the node that issues a 6P request; node B the node that receives this request.
We consider that node A monitors the communication cells it has in its schedule to node B:
If node A determines that the number of link-layer frames it is sending to node B per unit of time exceeds the capacity offered by the TSCH cells it has scheduled to node B, it triggers a 6P Transaction with node B to add one or more cells to the TSCH schedule of both nodes.
If the traffic is lower than the capacity, node A triggers a 6P Transaction with node B to delete one or more cells in the TSCH schedule of both nodes.
Node A MAY also monitor statistics to determine whether collisions are happening on a particular cell to node B.
If this feature is enabled, node A communicates with node B to "relocate" the cell which undergoes collisions to a different [slotOffset,channelOffset] location in the TSCH schedule.
This results in distributed schedule management in a 6TiSCH network.
The 6top Scheduling Function (SF) defines when to add/delete a cell to a neighbor.
Different applications require different SFs, so the SF is left out of scope of this document.
Different SFs are expected to be defined in future companion specifications.
A node MAY implement multiple SFs and run them at the same time.
At least one SF MUST be running.
The SFID field contained in all 6P messages allows a node to invoke the appropriate SF on a per-6P Transaction basis.
describes the 6TiSCH Operation Sublayer (6top).
defines the 6top Protocol (6P).
provides guidelines on how to define an SF.
As depicted in , the 6TiSCH Operation Sublayer (6top) is the next higher layer to the IEEE Std 802.15.4 TSCH medium access control (MAC) layer .
We use "802.15.4" as a short version of "IEEE Std 802.15.4" in this document.
The roles of the 6top sublayer are to:
Terminate the 6top Protocol (6P), which allows neighbor nodes to communicate to add/delete cells to one another.Run one or multiple 6top Scheduling Functions (SFs), which define the rules that decide when to add/delete cells.
Each cell in the schedule is either "hard" or "soft":
a soft cell can be read, added, deleted or updated by 6top.a hard cell is read-only for 6top.
In the context of this specification, all the cells used by 6top are soft cells.
Hard cells can be used for example when "hard-coding" a schedule .
6P MAY be used alongside the Minimal 6TiSCH Configuration .
In this case, it is RECOMMENDED to use 2 slotframes, as depicted in :
Slotframe 0 is used for traffic defined in the Minimal 6TiSCH Configuration.
In , Slotframe 0 is 5 slots long, but it can be shorter or longer.
6P allocates cells from Slotframe 1.
In , Slotframe 1 is 10 slots long, but it can be shorter or longer.
B| | | | | | |B->A| |
(6P) | | | | | | | | | | |
+-------------------------------------------------+
]]>
The Minimal 6TiSCH Configuration cell SHOULD be allocated from a slotframe of higher priority than the slotframe used by 6P for dynamic cell allocation.
This way, dynamically allocated cells cannot "mask" the cells used by the Minimal 6TiSCH Configuration.
6top MAY support additional slotframes; how to use additional slotframes is out of scope for this document.
The 6top Protocol (6P) enables two neighbor nodes to add/delete/relocate cells in their TSCH schedule.
Conceptually, two neighbor nodes "negotiate" the location of the cells to add, delete, or relocate in their TSCH schedule.
We call "6P Transaction" a complete negotiation between two neighbor nodes.
A particular 6P Transaction is executed between two nodes as a result of an action triggered by one SF.
For a 6P Transaction to succeed, both nodes must use the same SF to handle the particular transaction.
A 6P Transaction starts when a node wishes to add/delete/relocate one or more cells with one of its neighbors.
A 6P Transaction ends when the cell(s) have been added/deleted/relocated in the schedule of both nodes, or when the 6P Transaction has failed.
6P messages exchanged between nodes A and B during a 6P Transaction SHOULD be exchanged on non-shared unicast cells ("dedicated" cells) between A and B.
If no dedicated cells are scheduled between nodes A and B, shared cells MAY be used.
Keeping consistency between the schedules of the two neighbor nodes is important.
A loss of consistency can cause loss of connectivity.
One example is when node A has a transmit cell to node B, but node B does not have the corresponding reception cell.
To verify consistency, neighbor nodes maintain a Sequence Number (SeqNum).
Neighbor nodes exchange the SeqNum as part of each 6P Transaction to detect a possible inconsistency.
This mechanism is explained in .
An implementation MUST include a mechanism to associate each scheduled cell with the SF that scheduled it.
This mechanism is implementation-specific and out of scope of this document.
A 6P Transaction can consist of 2 or 3 steps.
A 2-step transaction is used when node A selects the cells to be allocated.
A 3-step transaction is used when node B selects the cells to be allocated.
An SF MUST specify whether to use 2-step transactions, 3-step transactions, or both.
We illustrate 2-step and 3-step transactions using the topology in .
shows an example 2-step 6P Transaction.
In a 2-step transaction, node A selects the candidate cells.
Several elements are left out to simplify understanding.
|
| | L2 ACK |
| 6P Timeout | X ||
In this example, the 2-step transaction occurs as follows:
The SF running on node A determines that 2 extra cells need to be scheduled to node B.
The SF running on node A selects candidate cells for node B to choose from.
Node A MUST select at least as many candidate cells as the number of cells to add.
Here, node A selects 3 candidate cells.
Node A locks those candidate cells in its schedule until it receives a 6P response.
Node A sends a 6P ADD Request to node B, indicating it wishes to add 2 cells (the "NumCells" value), and specifying the list of 3 candidate cells (the "CellList" value).
Each cell in the CellList is a [slotOffset,channelOffset] tuple.
This 6P ADD Request is link-layer acknowledged by node B (labeled "L2 ACK" in ).
After having successfully sent the 6P ADD Request (i.e. receiving the link-layer acknowledgment), node A starts a 6P Timeout to abort the 6P Transaction in case no response is received from node B.
The SF running on node B selects 2 out of the 3 cells from the CellList of the 6P ADD Request.
Node B locks those cells in its schedule until the transmission is successful (i.e. node B receives a link-layer ACK from node A).
Node B sends back a 6P Response to node A, indicating the cells it has selected.
The response is link-layer acknowledged by node A.
Upon completion of this 6P Transaction, 2 cells from A to B have been added to the TSCH schedule of both nodes A and B.
An inconsistency in the schedule can happen
if the 6P Timeout expires when the 6P Response is in the air,
if the last link-layer ACK for the 6P Response is lost, or
if one of the nodes is power cycled during the transaction.
6P provides an inconsistency detection mechanism described in to cope with such situations.
shows an example 3-step 6P Transaction.
In a 3-step transaction, node B selects the candidate cells.
Several elements are left out to simplify understanding.
|
| L2 ACK |
6P Timeout || 6P Timeout |
| | | |
| 6P Confirmation | | |
| Type = CONFIRMATION | | |
| Code = RC_SUCCESS | | |
cells | SeqNum = 178 | | |
locked | CellList = [(2,2),(3,5)] | | |
+-- |-------------------------------------->| X |
In this example, the 3-step transaction occurs as follows:
The SF running on node A determines that 2 extra cells need to be scheduled to node B.
The SF uses a 3-step transaction, so it does not select candidate cells.
Node A sends a 6P ADD Request to node B, indicating it wishes to add 2 cells (the "NumCells" value), with an empty "CellList".
This 6P ADD Request is link-layer acknowledged by node B.
After having successfully sent the 6P ADD Request, node A starts a 6P Timeout to abort the transaction in case no 6P Response is received from node B.
The SF running on node B selects 3 candidate cells, and locks them.
Node B sends back a 6P Response to node A, indicating the 3 cells it has selected.
The response is link-layer acknowledged by node A.
After having successfully sent the 6P Response, node B starts a 6P Timeout to abort the transaction in case no 6P Confirmation is received from node A.
The SF running on node A selects 2 cells from the CellList field in the 6P Response, and locks those.
Node A sends back a 6P Confirmation to node B, indicating the cells it selected.
The confirmation is link-layer acknowledged by node B.
Upon completion of the 6P Transaction, 2 cells from A to B have been added to the TSCH schedule of both nodes A and B.
An inconsistency in the schedule can happen
if the 6P Timeout expires when the 6P Confirmation is in the air,
if the last link-layer ACK for the 6P Confirmation is lost, or
if one of the nodes is power cycled during the transaction.
6P provides an inconsistency detection mechanism described in to cope with such situations.
6P messages travel over a single hop.
6P messages are carried as payload of an 802.15.4 Payload Information Element (IE) .
The messages are encapsulated within the Payload IE Header.
The Group ID is set to the IETF IE value defined in .
The content is encapsulated by a SubType ID, as defined in .
Since 6P messages are carried in IEs, IEEE bit/byte ordering applies.
Bits within each field in the 6top IE are numbered from 0 (leftmost and least significant) to k-1 (rightmost and most significant), where the length of the field is k bits.
Fields that are longer than a single octet are copied to the packet in the order from the octet containing the lowest numbered bits to the octet containing the highest numbered bits (little endian).
This document defines the "6top IE", a SubType of the IETF IE defined in , with subtype ID IANA_6TOP_SUBIE_ID.
The SubType Content of the "6top IE" is defined in .
The length of the "6top IE" content is variable.
All 6P messages follow the generic format shown in .
The version of the 6P protocol.
Only version 0 is defined in this document.
Future specifications may define further versions of the 6P protocol.
Type of message.
The message types are defined in .
Reserved bits.
These two bits SHOULD be set to zero when sending the message, and MUST be ignored upon reception.
The Code field contains a 6P Command Identifier when the 6P message is of Type REQUEST.
lists the 6P command identifiers.
The Code field contains a 6P return code when the 6P message is of Type RESPONSE or CONFIRMATION.
lists the 6P return codes.
The same return codes are used in both 6P Response and 6P Confirmation messages.
The identifier of the SF to use to handle this message.
The SFID is defined in .
Sequence number associated with the 6P Transaction, used to match the
6P Request,
6P Response and
6P Confirmation of the same 6P Transaction.
The value of SeqNum MUST be different at each new 6P Request issued to the same neighbor and using the same SF.
The SeqNum is also used to ensure consistency between the schedules of the two neighbors.
details how the SeqNum is managed.
The list of other fields and how they are used is detailed in .
6P Requests, 6P Response and 6P Confirmation messages for a same transaction MUST share the same Version, SFID and SeqNum values.
Future versions of the 6P Message SHOULD maintain the format of the 6P Version, Type and Code fields for backward compatibility.
An 8-bit 6P CellOptions bitmap is present in the following 6P requests: ADD, DELETE, COUNT, LIST, RELOCATE.
The format and meaning of this field MAY be redefined by the SF; the routine that parses this field is therefore associated with a specific SF.
In the 6P ADD request, the 6P CellOptions bitmap is used to specify what type of cell to add.In the 6P DELETE request, the 6P CellOptions bitmap is used to specify what type of cell to delete.In the 6P RELOCATE request, the 6P CellOptions bitmap is used to specify what type of cell to relocate.In the 6P COUNT and
the 6P LIST requests, the 6P CellOptions bitmap is used as a selector of a particular type of cells.
The content of the 6P CellOptions bitmap applies to all elements in the CellList field.
The possible values of the 6P CellOptions are:
TX = 1 (resp. 0) refers to macTxType = TRUE (resp. FALSE) in the macLinkTable of 802.15.4 .
RX = 1 (resp. 0) refers to macRxType = TRUE (resp. FALSE) in the macLinkTable of 802.15.4.
S = 1 (resp. 0) refers to macSharedType = TRUE (resp. FALSE) in the macLinkTable of 802.15.4.
contains the format of the 6P CellOptions bitmap,
unless redefined by the SF.
contains the meaning of the 6P CellOptions bitmap for the 6P ADD, DELETE, RELOCATE requests,
unless redefined by the SF.
contains the meaning of the 6P CellOptions bitmap for the 6P COUNT, LIST requests,
unless redefined by the SF.
The CellOptions is an opaque set of bits, sent unmodified to the SF.
The SF MAY redefine the format and meaning of the CellOptions field.
A CellList field MAY be present in
a 6P ADD Request,
a 6P DELETE Request,
a 6P RELOCATE Request,
a 6P Response, or
a 6P Confirmation.
It is composed of a concatenation of zero, one or more 6P Cells as defined in .
The content of the CellOptions field specifies the options associated with all cells in the CellList.
This necessarily means that the same options are associated with all cells in the CellList.
A 6P Cell is a 4-byte field, its default format is:
The slot offset of the cell.
The channel offset of the cell.
The CellList is an opaque set of bytes, sent unmodified to the SF.
The length of the CellList field is implicit, and determined by the IE Length field of the Payload IE header as defined in 802.15.4.
The SF MAY redefine the format of the CellList field; the routine that parses this field is therefore associated with a specific SF.
Cells are added by using the 6P ADD command.
The Type field (T) is set to REQUEST.
The Code field is set to ADD.
defines the format of a 6P ADD Request.
Used as extra signaling to the SF.
The contents of the Metadata field is an opaque set of bytes passed unmodified to the SF.
The meaning of this field depends on the SF, and is out of scope of this document.
For example, Metadata can specify in which slotframe to add the cells.
Indicates the options to associate with the cells to be added.
If more than one cell is added (NumCells>1), the same options are associated with each one.
This necessarily means that, if node A needs to add multiple cells with different options, it needs to initiate multiple 6P ADD Transactions.
The number of additional cells node A wants to schedule to node B.
A list of 0 or multiple candidate cells.
Its length is implicit and determined by the Length field of the Payload IE header.
defines the format of a 6P ADD Response and Confirmation.
A list of 0 or more 6P Cells.
Consider the topology in where the SF on node A decides to add NumCells cells to node B.
Node A's SF selects NumCandidate cells from its schedule.
These are cells that are candidates to be scheduled with node B.
The CellOptions field specifies the type of these cells.
NumCandidate MUST be larger or equal to NumCells.
How many cells node A selects (NumCandidate) and how that selection is done is specified in the SF and out of scope of this document.
Node A sends a 6P ADD Request to node B which contains
the CellOptions,
the value of NumCells, and
a selection of NumCandidate cells in the CellList.
In case the NumCandidate cells do not fit in a single packet, this operation MUST be split into multiple independent 6P ADD Requests, each for a subset of the number of cells that eventually need to be added.
In case of a 3-step transaction, the SF is responsible for ensuring that the returned candidate CellList fits into the 6P Response.
Upon receiving the request, node B checks whether the cellOptions are set to a valid value as noted by .
If this is not the case, a Response with code RC_ERR is returned.
If the cells in the received CellList in node B is smaller than NumCells, Node B MUST return a 6P Response with RC_ERR_CELLLIST code.
Otherwise, node B's SF verifies which of the cells in the CellList it can install in node B's schedule, following the specified CellOptions field.
How that selection is done is specified in the SF and out of scope of this document.
The verification can
succeed (NumCells cells from the CellList can be used),
fail (none of the cells from the CellList can be used), or
partially succeed (fewer than NumCells cells from the CellList can be used).
In all cases, node B MUST send a 6P Response with return code set to RC_SUCCESS, and which specifies the list of cells that were scheduled following the CellOptions field.
That can contain
NumCells elements (succeed),
0 elements (fail), or
between 0 and NumCells elements (partially succeed).
Upon receiving the response, node A adds the cells specified in the CellList according to the CellOptions field.
Cells are deleted by using the 6P DELETE command.
The Type field (T) is set to REQUEST.
The Code field is set to DELETE.
defines the format of a 6P DELETE Request.
Same usage as for the 6P ADD command, see .
Its format is the same as that in the 6P ADD command, but its content could be different.
Indicates the options that need to be associated to the cells to delete.
Only cells matching the CellOptions can are deleted.
The number of cells from the specified CellList the sender wants to delete from the schedule of both sender and receiver.
A list of 0 or more 6P Cells.
Its length is determined by the Length field of the Payload IE header.
defines the format of a 6P DELETE Response and Confirmation.
A list of 0 or more 6P Cells.
The behavior for deleting cells is equivalent to that of adding cells except that:
The nodes delete the cells they agree upon rather than adding them.
All cells in the CellList MUST already be scheduled between the two nodes and MUST match the CellOptions field.
If node A puts cells in its CellList that are not already scheduled between the two nodes and match the CellOptions field, node B MUST reply with a RC_ERR_CELLLIST return code.
The CellList in a 6P Request (2-step transaction) or 6P Response (3-step transaction) MUST either be empty, contain exactly NumCells cells, or more than NumCells cells.
The case where the CellList is not empty but contains fewer than NumCells cells is not supported.
RC_ERR_CELLLIST code MUST be returned when the CellList contains fewer than NumCells cells.
If the CellList is empty, the SF on the receiving node SHOULD choose NumCells cells with the sender from its schedule, which match the CellOption field, and delete them.
If the CellList contains more than NumCells cells, the SF on the receiving node chooses exactly NumCells cells from the CellList to delete.
Cell relocation consists in moving a cell to a different [slotOffset,channelOffset] location in the schedule.
The Type field (T) is set to REQUEST.
The Code is set to RELOCATE.
defines the format of a 6P RELOCATE Request.
Same usage as for the 6P ADD command, see .
Indicates the options that need to be associated with cells to be relocated.
The number of cells to relocate, which MUST be equal or greater than 1.
The list of NumCells 6P Cells to relocate.
A list of NumCandidate candidate cells for node B to pick from.
NumCandidate MUST be 0, equal to NumCells, or greater than NumCells.
Its length is determined by the Length field of the Payload IE header.
In a 2-step 6P RELOCATE Transaction,
node A specifies both the cells it needs to relocate,
and the list of candidate cells to relocate to.
The Relocation CellList MUST contain exactly NumCells entries.
The Candidate CellList MUST contain at least NumCells entries (NumCandidate>=NumCells).
In a 3-step 6P RELOCATE Transaction,
node A specifies only the cells it needs to relocate,
but not the list of candidate cells to relocate to.
The Candidate CellList MUST therefore be empty.
defines the format of a 6P RELOCATE Response and Confirmation.
A list of 0 or more 6P Cells.
Node A's SF wants to relocate NumCells cells.
Node A creates a 6P RELOCATE Request, and indicates the cells it wants to relocate in the Relocation CellList.
It also selects NumCandidate cells from its schedule as candidate cells to relocate the cells to, and puts those in the Candidate CellList.
The CellOptions field specifies the type of the cell(s) to relocate.
NumCandidate MUST be larger or equal to NumCells.
How many cells it selects (NumCandidate) and how that selection is done is specified in the SF and out of scope of this document.
Node A sends the 6P RELOCATE Request to node B.
Upon receiving the request, Node B checks if the length of the Candidate CellList is larger or equal to NumCells.
Node B's SF verifies that all the cells in the Relocation CellList are scheduled with node A, and are associate the options specified in the CellOptions field.
If either check fails, node B MUST send a 6P Response to node A with return code RC_ERR_CELLLIST.
If both checks pass, node B's SF verifies which of the cells in the Candidate CellList it can install in its schedule.
How that selection is done is specified in the SF and out of scope of this document.
That verification on Candidate CellList can
succeed (NumCells cells from the Candidate CellList can be used),
fail (none of the cells from the Candidate CellList can be used) or
partially succeed (fewer than NumCells cells from the Candidate CellList can be used).
In all cases, node B MUST send a 6P Response with return code set to RC_SUCCESS, and which specifies the list of cells that will be re-scheduled following the CellOptions field.
That can contain
NumCells elements (succeed),
0 elements (fail),
between 0 and NumCells elements (partially succeed).
If N < NumCells cells appear in the CellList, this means the first N cells in the Relocation CellList have been relocated, the remainder have not.
Upon receiving the response with Code RC_SUCCESS, node A relocates the cells specified in Relocation CellList of its RELOCATE Request to the new locations specified in the CellList of the 6P Response, in the same order.
In case the received return code is RC_ERR_CELLLIST, the transaction is aborted and no cell is relocated.
In case of a 2-step transaction, Node B relocates the selected cells upon receiving the link-layer ACK for the 6P Response.
In case of a 3-step transaction, Node B relocates the selected cells upon receiving the 6P Confirmation.
The SF SHOULD NOT relocate all cells between two nodes at the same time, which might result in the schedules of both nodes diverging significantly.
shows an example of a successful 2-step 6P RELOCATION Transaction.
| B prepares
| L2 ACK | to relocate
|(5,3)
| | and
| | (2,2)->(3,3)
| 6P Response |
| Code = RC_SUCCESS |
| SeqNum = 11 |
| CellList = [(5,3),(3,3)] |
A relocates|(5,3)| L2 ACK |
and | - - - - - - - - - - - - - - - - - - ->|B relocates
(2,2)->(3,3)| |(1,2)->(5,3)
| |and
| |(2,2)->(3,3)
]]> shows an example of a partially successful 2-step 6P RELOCATION Transaction.
|to relocate
| L2 ACK |(1,2)->(4,3)
|(4,3)| L2 ACK |
| - - - - - - - - - - - - - - - - - - ->|B relocates
| |(1,2)->(4,3)
| |
| |
]]> shows an example of a failed 2-step 6P RELOCATION Transaction.
| B cannot
| L2 ACK | relocate
||
relocate | |
| |
]]> shows an example of a successful 3-step 6P RELOCATION Transaction.
|
| L2 ACK |
|(5,3) | - - - - - - - - - - - - - - - - - - ->|
and | |
(2,2)->(3,3) | 6P Confirmation |
| Code = RC_SUCCESS |
| SeqNum = 11 |
| CellList = [(5,3),(3,3)] |
|-------------------------------------->| B relocates
| L2 ACK | (1,2)->(5,3)
A relocates |(5,3)| | (2,2)->(3,3)
and | |
(2,2)->(3,3)| |
| |
]]>
To retrieve the number of scheduled cells node A has with B, node A issues a 6P COUNT command.
The Type field (T) is set to REQUEST.
The Code field is set to COUNT.
defines the format of a 6P COUNT Request.
Same usage as for the 6P ADD command, see .
Its format is the same as that in the 6P ADD command, but its content could be different.
Specifies which type of cell to be counted.
defines the format of a 6P COUNT Response.
The number of cells which correspond to the fields of the request.
Node A issues a COUNT command to node B, specifying some cell options.
Upon receiving the 6P COUNT request,
node B goes through its schedule and counts the number of cells scheduled with node A in its own schedule
which match the cell options in the CellOptions field of the request.
details the use of the CellOptions field.
Node B issues a 6P response to node A with return code set to RC_SUCCESS,
and with NumCells containing the number of cells that match the request.
To retrieve a list of scheduled cells node A has with node B, node A issues a 6P LIST command.
The Type field (T) is set to REQUEST.
The Code field is set to LIST.
defines the format of a 6P LIST Request.
Same usage as for the 6P ADD command, see .
Its format is the same as that in the 6P ADD command, but its content could be different.
Specifies which type of cell to be listed.
Reserved bits.
These bits SHOULD be set to zero when sending the message, and MUST be ignored upon reception.
The Offset of the first scheduled cell that is requested.
The mechanism assumes cells are ordered according to a rule defined in the SF.
The rule MUST always order the cells in the same way.
The maximum number of cells to be listed.
Node B MAY return fewer than MaxNumCells cells, for example if MaxNumCells cells do not fit in the frame.
defines the format of a 6P LIST Response.
A list of 0 or more 6P Cells.
When receiving a LIST command, node B returns the cells scheduled with A in its schedule
that match the CellOptions field as specified in .
When node B receives a LIST request, the returned CellList in the 6P Response contains between 0 and MaxNumCells cells, starting from the specified offset.
Node B SHOULD include as many cells as fit in the frame.
If the response contains the last cell, Node B MUST set the Code field in the response to RC_EOL ("End of List", as per ), indicating to Node A that there no more cells that match the request.
Node B MUST return at least one cell, unless the specified Offset is beyond the end of B's cell list in its schedule.
If node B has fewer than Offset cells that match the request, node B returns an empty CellList and a Code field set to RC_EOL.
To clear the schedule between nodes A and B (for example after a schedule inconsistency is detected), node A issues a CLEAR command.
The Type field (T) is set to 6P Request.
The Code field is set to CLEAR.
defines the format of a 6P CLEAR Request.
Same usage as for the 6P ADD command, see .
Its format is the same as that in the 6P ADD command, but its content could be different.
defines the format of a 6P CLEAR Response.
When a 6P CLEAR command is issued from node A to node B, both nodes A and B MUST remove all the cells scheduled between them.
That is, node A MUST remove all the cells scheduled with node B, and
node B MUST remove all the cells scheduled with node A.
In a 6P CLEAR command, the SeqNum MUST NOT be checked.
In particular, even if the request contains a SeqNum value that would normally cause node B to detect a schedule inconsistency, the transaction MUST NOT be aborted.
Upon 6P CLEAR completion, the value of SeqNum MUST be reset to 0.
The return code to a 6P CLEAR command SHOULD be RC_SUCCESS unless the operation cannot be executed.
When the CLEAR operation cannot be executed, the return code MUST be set to RC_RESET.
The 6P SIGNAL message allows the SF implementations on two neighbor nodes to exchange generic commands.
The payload in a received SIGNAL message is an opaque set of bytes passed unmodified to the SF.
The length of the payload is determined through the length field of the Payload IE Header.
How the generic SIGNAL command is used is specified by the SF, and outside the scope of this document.
The Type field (T) is set to REQUEST.
The Code field is set to SIGNAL.
defines the format of a 6P SIGNAL Request.
Same usage as for the 6P ADD command, see .
Its format is the same as that in the 6P ADD command, but its content could be different.
defines the format of a 6P SIGNAL Response.
All messages contain a Version field.
If multiple Versions of the 6P protocol have been defined (in future specifications for Version values different from 0), a node MAY implement multiple protocol versions at the same time.
When a node receives a 6P message with a Version number it does not implement, the node MUST reply with a 6P Response with a return code field set to RC_ERR_VERSION.
The format of this 6P Response message MUST be compliant with Version 0 and MUST be supported by all future versions of the protocol.
This ensures that, when node B sends a 6P Response to node A indicating it does not implement the 6P version in the 6P Request, node A can successfully parse that response.
When a node supports a version number received in a 6P Request message,
the Version field in the 6P Response MUST be the same as the Version field in the corresponding 6P Request.
Similarly, in a 3-step transaction, the Version field in the 6P Confirmation MUST match that of the 6P Request and 6P Response of the same transaction.
All messages contain an SFID field.
A node MAY support multiple SFs at the same time.
When receiving a 6P message with an unsupported SFID, a node MUST reply with a 6P Response with return code of RC_ERR_SFID.
The SFID field in the 6P Response MUST be the same as the SFID field in the corresponding 6P Request.
In a 3-step transaction, the SFID field in the 6P Confirmation MUST match that of the 6P Request and the 6P Response of the same transaction.
Only a single 6P Transaction between two neighbors, in a given direction, can take place at the same time.
That is, a node MUST NOT issue a new 6P Request to a given neighbor before the previous 6P Transaction it initiated has finished (possibly timed out).
If a node receives a 6P Request from a given neighbor before having sent the 6P Response to the previous 6P Request from that neighbor, it MUST send back a 6P Response with a return code of RC_RESET (as per ) and discard this ongoing second transaction.
A node receiving a RC_RESET code MUST abort the second transaction and consider it never happened (i.e. reverting changes to the schedule or SeqNum done by this transaction).
Nodes A and B MAY support having two transactions going on at the same time, one in each direction.
Similarly, a node MAY support concurrent 6P Transactions with different neighbors.
In this case, the cells involved in an ongoing 6P Transaction MUST be "locked" until the transaction finishes.
For example, in , node C can have a different ongoing 6P Transaction with nodes B and R.
In case a node does not have enough resources to handle concurrent 6P Transactions from different neighbors it MUST reply with a 6P Response with return code RC_ERR_BUSY (as per ).
In case the requested cells are locked, it MUST reply to that request with a 6P Response with return code RC_ERR_LOCKED (as per ).
The node receiving RC_ERR_BUSY or a RC_ERR_LOCKED MAY implement a retry mechanism, defined by the SF.
A timeout occurs when the node that successfully sent a 6P Request does not receive the corresponding 6P Response within an amount of time specified by the SF.
In a 3-step transaction, a timeout also occurs when a node sending the 6P Response does not receive a 6P Confirmation.
When a timeout occurs, the transaction MUST be canceled at the node where the timeout occurs.
The value of the 6P Timeout should be larger than the longest possible time it takes to receive the 6P Response or Confirmation.
The value of the 6P Timeout hence depends on
the number of cells scheduled between the neighbor nodes,
the maximum number of link-layer retransmissions, etc.
The SF MUST determine the value of the timeout.
The value of the timeout is out of scope of this document.
In case the receiver of a 6P Request fails during a 6P Transaction and is unable to complete it, it SHOULD reply to that Request with a 6P Response with return code RC_RESET.
Upon receiving this 6P Response, the initiator of the 6P Transaction MUST consider the 6P Transaction as failed.
Similarly, in the case of 3-step transaction, when the receiver of a 6P Response fails during the 6P Transaction and is unable to complete it, it MUST reply to that 6P Response with a 6P Confirmation with return code RC_RESET.
Upon receiving this 6P Confirmation, the sender of the 6P Response MUST consider the 6P Transaction as failed.
The SeqNum is the field in the 6top IE header used to match Request, Response and Confirmation.
The SeqNum is used to detect and handle
duplicate commands () and
schedule inconsistencies ().
Each node remembers the last used SeqNum for each neighbor.
That is, a node stores as many SeqNum values as it has neighbors.
In case of supporting multiple SFs at a time, a SeqNum value is maintained per SF and per neighbor.
In the remainder of this section, we describe the use of SeqNum between two neighbors;
the same happens for each other neighbor, independently.
When a node resets or after a CLEAR transaction, it MUST reset SeqNum to 0.
The 6P Response and 6P Confirmation for a transaction MUST use the same SeqNum value as that in the Request.
After every transaction, the SeqNum MUST be incremented by exactly 1.
Specifically, if node A receives the link-layer acknowledgment for its 6P Request, it commits to incrementing the SeqNum by exactly 1 after the 6P Transaction ends.
This ensure that, at the next 6P Transaction where it sends a 6P Request, 6P Request will have a different SeqNum.
Similarly, a node B increments the SeqNum by exactly 1
after having received the link-layer acknowledgment for the 6P Response (2-step 6P Transaction), or
after having sent the link-layer acknowledgment for the 6P Confirmation (3-step 6P Transaction)
.
When a node B receives a 6P Request from node A with SeqNum equal to 0, it checks the stored SeqNum for A.
If A is a new neighbor, the stored SeqNum in B will be 0.
The transaction can continue.
If the stored SeqNum for A in B is different than 0, a potential inconsistency is detected.
In this case, B MUST return RC_ERR_SEQNUM with SeqNum=0.
The SF of node A MAY decide what to do next, as described in .
The SeqNum MUST be implemented as a lollipop counter: it rolls over from 0xFF to 0x01 (not to 0x00).
This is used to detect a neighbor reset.
lists the possible values of the SeqNum.
All 6P commands are link-layer acknowledged.
A duplicate message means that a node receives a second 6P Request, Response or Confirmation.
This happens when the link-layer acknowledgment is not received, and a link-layer retransmission happens.
Duplicate messages are normal and unavoidable.
shows an example 2-step transaction in which Node A receives a duplicate 6P Response.
|
| L2 ACK |
||
| |
]]> shows example 3-step transaction in which Node A receives a out-of-order duplicate 6P Response after having sent a 6P Confirmation.
|
| L2 ACK |
|| |
| L2 ACK | |
||
received | |
]]>
A node detects a duplicate 6P message when it has the same SeqNum and type as the last frame received from the same neighbor.
When receiving a duplicate 6P message, a node MUST send a link-layer acknowledgment, but MUST silently ignore the 6P message at the 6top sublayer.
A schedule inconsistency happens when the schedules of nodes A and B are inconsistent.
For example, when node A has a transmit cell to node B, but node B does not have the corresponding receive cell, and therefore isn't listening to node A on that cell.
A schedule inconsistency results in loss of connectivity.
The SeqNum field, which is present in each 6P message, is used to detect an inconsistency.
The SeqNum field increments by 1 at each message, as detailed in .
A node computes the expected SeqNum field for the next 6P Transaction.
If a node receives a 6P Request with a SeqNum value that is not the expected one, it has detected an inconsistency.
There are at least 2 cases in which a schedule inconsistency happens.
The first case is when a node loses state, for example when it is power cycled (turned off, then on).
In that case, its SeqNum value is reset to 0.
Since the SeqNum is a lollipop counter, its neighbor detects an inconsistency at the next 6P transaction.
This is illustrated in and .
|
| L2 ACK |
||
| ==== power-cycle
| |
SeqNum=88 | | SeqNum=0
| |
| 6P Request (SeqNum=88) |
|-------------------------------------->| Inconsistency
| L2 ACK | Detected
||
]]>|
| L2 ACK |
||
| ==== power-cycle
| |
SeqNum=98 | | SeqNum=0
| |
| 6P Request (SeqNum=0) |
Inconsistency||
| |
| 6P Response (SeqNum=0, RC_ERR_SEQNUM) |
|-------------------------------------->|
| L2 ACK |
|
The second case is when the maximum number of link-layer retransmissions is reached on the 6P Response of a 2-step transaction (or equivalently on a 6P Confirmation of a 3-step transaction).
This is illustrated in .
|
| L2 ACK |
|
In both cases, node B detects the inconsistency.
If the inconsistency is detected during a 6P Transaction (), the node that has detected it MUST send back a 6P Response or 6P Confirmation with an error code of RC_ERR_SEQNUM.
In this 6P Response or 6P Confirmation, the SeqNum field MUST be set to the value of the sender of the message (0 in the example in ).
The SF of the node which has detected the inconsistency MUST define how to handle the inconsistency.
A first possibility is to issue a 6P CLEAR request to clear the schedule, and rebuild.
A second possibility is to issue a 6P LIST request to retrieve the schedule.
A third possibility is to internally "roll-back" the schedule.
How to handle an inconsistency is out of scope of this document.
The SF defines how to handle an inconsistency.
A return code marked as Yes in the "Is Error" column in indicates an error.
When a node receives a 6P Response or 6P Confirmation with an error, it MUST consider the 6P Transaction as failed.
In particular,
if this was a response to a 6P ADD, DELETE or RELOCATE Request,
the node MUST NOT add, delete or relocate any of the cells involved in this 6P Transaction.
Similarly, a node sending a 6P Response or a 6P Confirmation with an error code MUST NOT add, delete, relocate any cells as part of that 6P Transaction.
If a node receives an unrecognized return code the 6P Transaction MUST be considered as failed.
In particular, in a 3 step 6P Transaction, a 6P Response with an unrecognized return code MUST be responded with a 6P Confirmation with return code RC_ERR and consider the transaction as failed.
Defining what to do after an error has occurred is out of scope of this document.
The SF defines what to do after an error has occurred.
6P messages MUST be secured through link-layer security.
This is possible because 6P messages are carried as Payload IEs.
Each SF has a 1-byte identifier.
defines the rules for applying for an SFID.
The specification for an SF
MUST specify an identifier for that SF.MUST specify the rule for a node to decide when to add/delete one or more cells to a neighbor.MUST specify the rule for a Transaction source to select cells to add to the CellList field in the 6P ADD Request.MUST specify the rule for a Transaction destination to select cells from CellList to add to its schedule.MUST specify a value for the 6P Timeout, or a rule/equation to calculate it.MUST specify the rule for ordering cells.MUST specify a meaning for the "Metadata" field in the 6P ADD Request.MUST specify the SF behavior of a node when it boots.MUST specify how to handle a schedule inconsistency.MUST specify what to do after an error has occurred (either the node sent a 6P Response with an error code, or received one).
MUST specify the list of statistics to gather.
Example statistics include the number of transmitted frames to each neighbor.
In case the SF requires no statistics to be gathered, the specific of the SF MUST explicitly state so.
SHOULD clearly state the application domain the SF is created for.SHOULD contain examples which highlight normal and error scenarios.SHOULD contain a list of current implementations, at least during the I-D state of the document, per .SHOULD contain a performance evaluation of the scheme, possibly through references to external documents.SHOULD define the format of the SIGNAL command payload and its use.MAY redefine the format of the CellList field.MAY redefine the format of the CellOptions field.MAY redefine the meaning of the CellOptions field.
6P messages are carried inside 802.15.4 Payload Information Elements (IEs).
Those Payload IEs are encrypted and authenticated at the link layer through CCM* .
6P benefits from the same level of security as any other Payload IE.
The 6P protocol does not define its own security mechanisms.
In particular, although a key management solution is out of scope of this document, the 6P protocol will benefit for the key management solution used in the network.
This is relevant as security attacks such as forgery and misattribution attacks become more damaging when a single key is shared amongst a group of more than 2 participants.
The 6P protocol does not provide protection against DOS attacks.
Example attacks include,
not sending confirmation messages in 3-step transaction, and
sending wrongly formatted requests.
These cases SHOULD be handled by an appropriate policy, such as rate-limiting or time-limited blacklisting the attacker after several attempts.
The effect on the overall network is mostly localized to those two nodes, as communication happens in dedicated cells.
This document adds the following number to the "IEEE Std 802.15.4 IETF IE subtype IDs" registry defined by :
| SUBID_6TOP | RFCXXXX |
+---------------------+-----------+
]]>
This section defines sub-registries within the "IPv6 over the TSCH mode of IEEE 802.15.4e (6TiSCH) parameters" registry, hereafter referred to as the "6TiSCH parameters" registry.
Each sub-registry is described in a subsection.
The name of the sub-registry is "6P Version Numbers".
A Note included in this registry should say:
"In the 6top Protocol (6P) [RFCXXXX] there is a field to identify the version of the protocol.
This field is 4 bits in size."
Each entry in the sub-registry must include
the Version in the range 0-15, and
a reference to the 6P version's documentation.
The initial entry in this sub-registry is as follows:
All other Version Numbers are Unassigned.
The IANA policy for future additions to this sub-registry is "IETF Review or IESG Approval" as described in .
The name of the sub-registry is "6P Message Types".
A note included in this registry should say:
"In the 6top Protocol (6P) version 0 [RFCXXXX], there is a field to identify the type of message.
This field is 2 bits in size."
Each entry in the sub-registry must include
the Type in range b00-b11,
the corresponding Name, and
a reference to the 6P message type's documentation.
Initial entries in this sub-registry are as follows:
All other Message Types are Reserved.
The IANA policy for future additions to this sub-registry is "IETF Review or IESG Approval" as described in .
The name of the sub-registry is "6P Command Identifiers".
A Note included in this registry should say:
"In the 6top Protocol (6P) version 0 [RFCXXXX], there is a Code field which is 8 bits in size.
In a 6P Request, the value of this Code field is used to identify the command."
Each entry in the sub-registry must include
an Identifier in the range 0-255,
the corresponding Name, and
a reference to the 6P command identifier's documentation.
Initial entries in this sub-registry are as follows:
The IANA policy for future additions to this sub-registry is "IETF Review or IESG Approval" as described in .
The name of the sub-registry is "6P Return Codes".
A Note included in this registry should say:
"In the 6top Protocol (6P) version 0 [RFCXXXX], there is a Code field which is 8 bits in size.
In a 6P Response or 6P Confirmation, the value of this Code field is used to identify the return code."
Each entry in the sub-registry must include
a Code in the range 0-255,
the corresponding Name,
the corresponding Description, and
a reference to the 6P return code's documentation.
Initial entries in this sub-registry are as follows:
All other Message Types are Unassigned.
The IANA policy for future additions to this sub-registry is "IETF Review or IESG Approval" as described in .
6P Scheduling Function Identifiers.
A Note included in this registry should say:
"In the 6top Protocol (6P) version 0 [RFCXXXX], there is a field to identify the scheduling function to handle the message.
This field is 8 bits in size."
Each entry in the sub-registry must include
an SFID in the range 0-255,
the corresponding Name, and
a reference to the 6P Scheduling Function's documentation.
Initial entries in this sub-registry are as follows:
All other Message Types are Unassigned.
The IANA policy for future additions to this sub-registry depends on the value of the SFID, as defined in .
These specifications must follow the guidelines of .
The name of the sub-registry is "6P CellOptions bitmap".
A Note included in this registry should say:
"In the 6top Protocol (6P) version 0 [RFCXXXX], there is an optional CellOptions field which is 8 bits in size."
Each entry in the sub-registry must include
a bit position in the range 0-7,
the corresponding Name, and
a reference to the bit's documentation.
Initial entries in this sub-registry are as follows:
All other Message Types are Reserved.
The IANA policy for future additions to this sub-registry is "IETF Review or IESG Approval" as described in .
IEEE Std 802.15.4-2015 - IEEE Standard for Low-Rate Wireless Personal Area Networks (WPANs)IEEE standard for Information TechnologyOpenWSN: a Standards-Based Low-Power Wireless Development Environment
Formal Specification of the CCM* Mode of Operation, IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs).
The following section structure for a SF document is RECOMMENDED:
IntroductionScheduling Function IdentifierRules for Adding/Deleting CellsRules for CellList6P Timeout ValueRule for Ordering CellsMeaning of the Metadata FieldNode Behavior at BootSchedule Inconsistency Handling6P Error HandlingExamplesImplementation StatusSecurity ConsiderationsIANA Considerations